Beneficial effects of oral and topical sodium bicarbonate during a battery of team sport-specific exercise tests in recreationally trained male athletes

ABSTRACT Objective This study examined the effects of oral and topical (PR Lotion; Momentous) sodium bicarbonate (NaHCO3) during a battery of team sport-specific exercise tests. Method In a block randomized, crossover, double-blind, placebo-controlled design, 14 recreationally trained male team sport athletes performed a familiarization visit and three experimental trials receiving: (i) 0.3 g·kg−1 body mass (BM) NaHCO3 in capsules + placebo lotion (SB-ORAL), (ii) placebo capsules +0.9036 g·kg−1 BM PR Lotion (SB-LOTION), or (iii) placebo capsules + placebo lotion (PLA). Supplements were given ~120 min prior to the team sport-specific exercise tests: countermovement jumps (CMJ), 8 × 25 m repeated sprints and Yo-Yo Intermittent Recovery Level 2 (Yo-Yo IR2). Blood acid–base balance (pH, bicarbonate) and electrolytes (sodium, potassium) were measured throughout. Rating of perceived exertion (RPE) was recorded after each sprint and post-Yo-Yo IR2. Results Distance covered during the Yo-Yo IR2 was 21% greater for SB-ORAL compared with PLA (+94 m; p = 0.009, d = 0.64) whereas performance was only 7% greater for SB-LOTION compared with PLA (480 ± 122 vs. 449 ± 110 m; p = 0.084). Total completion time for the 8 × 25 m repeated sprint test was 1.9% faster for SB-ORAL compared with PLA (−0.61 s; p = 0.020, d = 0.38) and 2.0% faster for SB-LOTION compared with PLA (−0.64 s; p = 0.036, d = 0.34). CMJ performance was similar between treatments (p > 0.05). Blood acid–base balance and electrolytes were significantly improved for SB-ORAL compared with PLA, but no differences were observed for SB-LOTION. Compared to PLA, RPE was lower for SB-LOTION after the fifth (p = 0.036), sixth (p = 0.012), and eighth (p = 0.040) sprints and for SB-ORAL after the sixth (p = 0.039) sprint. Conclusions Oral NaHCO3 improved 8 × 25 m repeated sprint (~2%) and Yo-Yo IR2 performance (21%). Similar improvements in repeated sprint times were observed for topical NaHCO3 (~2%), but no significant benefits were reported for Yo-Yo IR2 distance or blood acid–base balance compared to PLA. These findings suggest that PR Lotion might not be an effective delivery system for transporting NaHCO3 molecules across the skin and into systematic circulation, therefore further research is needed to elucidate the physiological mechanisms responsible for the ergogenic effects of PR Lotion.


Introduction
Athletes competing in field-based team sports repeatedly perform high-intensity efforts (i.e. sprints, jumps) interspersed with short rest periods [1]. Repeated sprint ability (RSA) is an important determinant of success in team sports but declines throughout competitive matches [2]. Substantial anaerobic energy demand from completing maximal sprint efforts leads to the accumulation of hydrogen ions (H + ) within muscles [3], which may cause an intramuscular acidosis. While the deleterious effects of declining pH during exercise are debated [4], a cellular acidosis is suggested to inhibit energy production via anaerobic glycolysis and limit action potentials required for muscle contractions [3,5,6]. These biochemical changes contribute toward skeletal muscle fatigue [7,8], therefore nutritional strategies that restore acid-base balance could prove beneficial during team sport exercise.
Sodium bicarbonate (NaHCO 3 ) is an extracellular buffering aid that has been extensively researched [9]. Ingestion of 0.3 g·kg −1 body mass (BM) NaHCO 3 in fluid or capsules 90-120 min pre-exercise raises the concentration of blood bicarbonate (HCO 3 − ) by ~5.0-6.0 mmol·L −1 , which elevates the pH gradient between intracellular and extracellular compartments, subsequently upregulating the lactate-H + co-transporter to remove H + from muscles [10]. Additionally, ingesting NaHCO 3 increases the amount of sodium (Na + ) in the blood [11,12], which may lead to improvements in hydration status [13]. Considering that NaHCO 3 ingestion may attenuate losses of intramuscular potassium (K + ) during exercise [12], these changes in electrolytes could also result in upregulation of Na + /K + -ATPase activity to limit muscle depolarization and sustain excitation-contraction coupling [8].
An established method for evaluating the ergogenic effect of NaHCO 3 on team sportspecific exercise performance is the Yo-Yo Intermittent Recovery Test Level 2 (Yo-Yo IR2) [14]. NaHCO 3 ingestion has improved distance covered during the Yo-Yo IR2 [15,16]. Similar results have been reported in team sport athletes for the effect of NaHCO 3 on RSA (3 sets, 6 × 20 m) [17]. Interestingly, others have shown no improvements for RSA in rugby [18] and soccer [19] players after 0.3 g·kg −1 BM NaHCO 3 . It is possible that gastrointestinal (GI) discomfort commonly associated with NaHCO 3 ingestion prevented these participants from improving their performance [18,20]. Orally ingested NaHCO 3 dissociates into Na + and HCO 3 − upon reaching the stomach, where the HCO 3 − neutralizes gastric acid, which generates excessive carbon dioxide production that can induce belching and vomiting [21]. These large amounts of exogenous Na + may also aggravate intestinal mucosa and prompt osmotic fluctuations that can cause diarrhea [18]. Many athletes are deterred from using NaHCO 3 due to these GI side effects, the poor palatability of fluid beverages and/or the high number of capsules (~35 for a 90 kg athlete) required to achieve an ergogenic dose [22]. NaHCO 3 supplementation strategies that bypass the GI tract could therefore provide a favorable alternative to oral ingestion [23]. One novel approach is PR Lotion (Momentous, Park City, Utah), which is a topically applied muscle cream that is purported to transport NaHCO 3 molecules across the skin via a transdermal drug delivery system [24]. PR Lotion adopts an innovative formulation that encapsulates NaHCO 3 molecules within fatty acid salts that fluidize the outermost layer of skin and modulate tight junctions in the epidermis, allowing NaHCO 3 molecules to be absorbed into the bloodstream [25]. McKay et al. [26] reported that PR Lotion did not significantly improve average power during 3 × 30 s Wingate cycling tests compared to a placebo (p = 0.108). However, they also found no differences in time-to-exhaustion (TTE) cycling performance between oral NaHCO 3 and PR Lotion (363 ± 80 vs. 349 ± 119 s, p = 0.697). Given their lack of a placebo group during the TTE cycling task, it is difficult to conclude whether there was an ergogenic effect of PR Lotion. Furthermore, it is not yet known whether PR Lotion alters the concentration of electrolytes and hydration status to a similar degree as oral NaHCO 3 ingestion. Additional research comparing oral and topical NaHCO 3 is needed before conclusions can be drawn regarding the efficacy of PR Lotion. Therefore, the aim of this study was to examine the effects of oral and topical NaHCO 3 (PR Lotion) during a battery of team sport-specific exercise tests. Our hypothesis was that oral and topical NaHCO 3 would improve team sport-specific exercise performance (countermovement jumps [CMJ], 8 × 25 m repeated sprints, Yo-Yo IR2) compared with a placebo.

Study design
A block randomized, double-blind, placebo-controlled, crossover design was used for this study. Participants were randomly allocated to receive each of the three nutritional supplements (oral NaHCO 3 , topical NaHCO 3 and placebo) in a counterbalanced order using an online sequence generator (www.randomization.com).

Participants
Our sample size calculation conducted on G*Power (version 3.1.9.4) revealed that 15 participants were needed to achieve statistical power (β = 0.80; α=0.05). This assumed that repeated measures analysis of variances (ANOVA; within-factors) would be used to analyze performance outcomes, with an expected medium effect size (η p 2 = 0.06).
Correlation between repeated measures was estimated from reliability data for our performance outcomes [27]. To account for 10-20% drop out rates, 18 participants were recruited; however, three withdrew due to injury and one because of time constraints. Therefore, 14 recreationally trained male team sport athletes (body mass: 81.9 ± 10.1 kg; stature: 182.0 ± 5.4 cm; age: 26.5 ± 5.8 years, sporting background: 3 × hockey, 7 × soccer, 2 × basketball, 1 × rugby, 1 × cricket) completed the study. Participants received a £25 gift voucher and free supplements after taking part.
Eligibility criteria stated that participants performed ≥2 training sessions per week of their sport. They were excluded if they had: (i) previously used NaHCO 3 , (ii) an intolerance to cornflour, and (iii) a medical condition that impacts high-intensity exercise. Ethical approval was obtained from the Institutional Ethics Committee (ER42014516). Participants completed a health questionnaire and provided written informed consent before commencing the study.

Procedures
Participants were instructed to avoid strenuous exercise and alcohol for 24 h prior to sessions. They attended the indoor sports hall on four separate occasions. The first visit was a familiarization session to habituate participants to the team sport-specific exercise tests. During the remaining three sessions, participants completed exercise tests after receiving either oral NaHCO 3 , topical NaHCO 3 or a placebo. These sessions were separated by 5-7 days to ensure appropriate recovery and washout of treatments. Testing was conducted at the same time of day to control for the confounding effect of circadian rhythms on exercise performance [28]. Participants were asked to wear the same footwear for each session and to replicate their diet 24 h prior to each visit.

Team sport-specific exercise tests
Participants completed a 10-min warm up of jogging, 3 × 10 m sprints, agility drills, stretches, and 3 × 20 m runs at 50%, 70%, and 90% of perceived maximum. After 5 min, they performed a first CMJ using an optimal measurement system (Optojump Next, Microgate, USA). Participants completed one practice and three maximal jumps separated by 1 min. After 3 min, they performed an 8 × 25 m repeated sprint test. Timing gates (Brower timing systems, Draper, Utah, USA) were placed at 0 and 25 m to record sprint times. Following each sprint, participants jogged back to the start line, with sprints departing every 25 s until the protocol had been repeated eight times. Participants received 2 min of recovery and then conducted a second CMJ test. After a further 5 min, they performed the Yo-Yo IR2, which required them to repeatedly carry out 2 × 20 m shuttle runs at increasing speeds dictated by audio signals [14]. Each stage was separated by 10 s recovery where participants jogged around a cone positioned 5 m behind the start line. Test termination was classified as two failures to reach the finish line before the signal, at which point distance covered during the Yo-Yo IR2 was recorded. After 5 min recovery, participants performed a final CMJ test.

Supplementation protocol
During experimental trials, participants performed the team sport-specific exercise tests after receiving: (i) NaHCO 3 in capsules + placebo lotion (SB-ORAL), (ii) placebo capsules + PR Lotion (SB-LOTION), and (iii) placebo capsules + placebo lotion (PLA). Supplements were prepared by a laboratory technician not involved with the study. Oral NaHCO 3 was given as a 0.3 g·kg −1 BM dose in size 0 vegetarian capsules (Your Supplements, Stockport, UK). An equal number of capsules (31 ± 4) containing cornflour was used as a placebo for SB-LOTION and PLA. Cornflour is an inert substance that effectively blinds NaHCO 3 [22].
Capsules were filled using a capsule filling device (ALL-IN Capsule, USA) and contained either ~0.8 g NaHCO 3 (Health Leads Ltd, UK) or ~0.4 g cornflour (Sainsbury's, UK). These were checked for weight and administered as three equal doses with 7 mL·kg −1 BM water at 15-min intervals across a 30-min period [29] commencing 135 min before the team sport-specific exercise tests. Participants applied PR Lotion to their legs and lower back as a 0.9036 g·kg −1 BM dose during the final 15 min of this 30-min supplementation period. PR Lotion was administered 120 min prior to the team sport-specific exercise tests as evidence suggests peak changes in muscle/blood pH occur ~120 min after applying PR Lotion [30]. PR Lotion is ~33% NaHCO 3 ; therefore, a 0.9036 g·kg −1 BM dose was chosen to theoretically match the amount of NaHCO 3 given during SB-ORAL and SB-LOTION. The placebo lotion was matched for all ingredients except NaHCO 3 and both lotions were provided in plastic tubs. Supplements were given alongside a carbohydrate-rich meal (1.5 g·kg −1 BM; biscuits, wholegrain cereal bars, cornflakes with milk) to standardize food consumed prior to testing and minimize GI discomfort after NaHCO 3 ingestion [29].

Experimental trials
Baseline urine samples were analyzed for color using an 8-point Likert scale [31] and osmolality using an osmometer (Vitech Scientific, Partridge Green, UK). Capillary blood samples (95 μL) were analyzed for acid-base balance (pH, HCO 3 ), electrolytes (Na + , K + ), hemoglobin, and hematocrit using a blood gas analyzer (i-STAT Alinity, Abbott, USA). Plasma volume was estimated from hemoglobin and hematocrit concentration [32]. Additional 20 μL blood samples were analyzed for lactate using a Biosen C-Line (EKF Diagnostics, Cardiff, UK). Visual analog scales (VAS) for eight GI side effects were completed to quantify aggregate GI discomfort [11,33]. Supplements were administered across a 30-min window. Participants then completed VAS and treatment assignment questionnaires that asked them to select which treatment they believed had been given ("oral NaHCO 3 ," "topical NaHCO 3 ," "placebo," "unsure") and explain their reasons [22]. Participants rested for a further 85 min, before blood and urine samples were taken pre-warm-up. They also repeated VAS and treatment assignment questionnaires and rated on 1-5 Likert type scales how much they expected the supplement they thought had been given to improve performance ("1" = no expectations, "5" = extremely high expectations).
After the 10-min warm-up, blood samples were repeated. Participants commenced the team sport-specific exercise tests 120 min after applying PR Lotion (~105 min after the final set of capsules). Additional blood samples were analyzed for lactate after CMJ tests and the repeated sprints. Rating of perceived exertion (RPE; 6-20 Borg scale) was recorded after each sprint and post-Yo-Yo IR2. Blood sampling was repeated pre-and post-Yo-Yo IR2. Post-exercise urine samples, treatment assignment questionnaires, and VAS were completed after the final CMJ test. An overview of experimental procedures is shown in Figure 1.

Statistical analysis
Statistical analyses were performed using SPSS 26.0 for Windows (IBM, Chicago, IL) and Exploratory Software for Confidence Intervals (ESCI, https://thenewstatistics. com/itns/esci/). Grouped data and standardized residuals were assessed for normality using Shapiro-Wilks tests. Homogeneity of variance/sphericity were analyzed using Mauchly tests and any violations were corrected via Greenhouse-Geisser adjustments. Two-way repeated measures ANOVA were used to determine treatment * time interactions for sprint completion times, CMJ performance, blood metabolites, urine osmolality and RPE during the repeated sprint test. All other normally distributed outcome measures were assessed using one-way repeated measures ANOVA. When significant effects were found, post hoc pairwise comparisons were made using Bonferroni correction factors. Effect sizes were calculated using SPSS for η p 2 and ESCI for Cohen's d. These were interpreted using the classifications of 0.01, 0.06, and 0.14 as small, medium, and large effect sizes for η p 2 and 0.2, 0.5, and 0.8 as small, moderate, and large effect sizes for Cohen's d [34]. Friedman tests were used to assess treatment effects for non-normally distributed data (GI discomfort, urine color, expectations), with Chi-square (χ 2 ) reported as the test statistic. When significant effects were found, post hoc pairwise comparisons were conducted, with median and Z values presented. Treatment assignment ratings ("correct," "incorrect") were analyzed using 2 × 2 χ 2 tests to determine blinding efficacy. Mean differences and 95% CI are reported for treatment comparisons. Data are presented as mean ± SD (unless stated) and statistical significance was set at p ≤0.05.

Team sport-specific exercise performance
There was an effect of treatment on Yo-Yo IR2 performance (p = 0.001, η p 2 = 0.414;

Hydration status
There were significant effects of treatment on changes in plasma volume from baseline to pre-warm-up (p < 0.001, η p 2 = 0.

Discussion
The aim of this study was to examine the effects of oral and topical NaHCO 3 during a battery of team sport-specific exercise tests. Topical NaHCO 3 (0.9036 g·kg −1 BM PR Lotion) improved 8 × 25 m repeated sprint performance by ~2% but had no significant effect on blood acid-base balance or electrolytes. Improvements in Yo-Yo IR2 and repeated sprint performance for oral NaHCO 3 were likely due to elevated extracellular buffering, increased glycolytic flux and sustained excitation-contraction coupling. We propose that the faster repeated sprint times for PR Lotion might be attributed to an interaction between NaHCO 3 molecules and menthol that intensified the localized cooling sensation, in turn reducing participants' perception of effort and allowing them to exert greater effort during exercise. Further research is required to replicate our findings for PR Lotion and elucidate the physiological mechanisms responsible for ergogenic effects. Yo-Yo IR2 performance improved by 21% for SB-ORAL compared to PLA, which represented a moderate effect and is congruent with evidence from a meta-analysis [35]. In contrast, SB-LOTION showed a small but statistically insignificant effect (7% greater total distance) compared to PLA. This is comparable to findings from McKay et al. [26] that demonstrated small, non-significant effects of PR Lotion during 3 × 30 s Wingate cycling tests and suggests that oral NaHCO 3 remains the most effective supplementation strategy for improving Yo-Yo IR2 performance. There were no improvements in CMJ performance for SB-ORAL or SB-LOTION, supporting some previous NaHCO 3 studies [19] but not others [36]. Discrepancies between studies could relate to differences in athlete cohort; we recruited a variety of team sport athletes meaning that there was a large variability in Table 3. RPE (au.) after each of the 8 × 25 m repeated sprints and post-Yo-Yo IR2. Values are presented as mean ± SD. RPE values were measured using the 6-20 Borg scale. SB-ORAL = oral sodium bicarbonate, SB-LOTION = topical sodium bicarbonate, PLA = placebo; * lower than PLA (p < 0.05).
CMJ performance that may have decreased the likelihood of observing any treatment effects. Interestingly, SB-ORAL and SB-LOTION improved 8 × 25 m repeated sprint performance (total and average times) by ~2%. These results add to equivocal previous findings for the effect of NaHCO 3 on RSA in team sport athletes [17][18][19] and provide evidence for the performance enhancing effects of PR Lotion during this exercise task. It is possible that we found improvements in repeated sprint performance for oral NaHCO 3 because of the absence of GI discomfort [20], as team sport athletes often experience severe side-effects [18]. They might be at greater risk of suffering GI discomfort than other athletes (i.e. cyclists) as their higher body mass requires larger absolute NaHCO 3 doses to achieve ergogenic benefits. PR Lotion could offer an alternative strategy for improving RSA in team sport athletes, but the effect on Yo-Yo IR2 and CMJ performance remains unclear.
Since this is the first study to show ergogenic effects of PR Lotion, it is important future work replicates our findings and investigates whether performance benefits exist for other exercise modalities. Pre-exercise changes in blood buffering capacity for SB-ORAL typically achieved the minimum ergogenic threshold (i.e. HCO 3 − >5.0 mmol·L −1 for 71% participants) [9] and blood acid-base balance was elevated compared to PLA throughout. Our improvements in repeated sprint and Yo-Yo IR2 performance for SB-ORAL were likely attributed to greater HCO 3 − buffering, which increased H + efflux from the muscle and protected against declining intramuscular pH [10]. This is supported by significantly higher blood lactate post-Yo-Yo IR2 for SB-ORAL, suggesting that NaHCO 3 ingestion may have upregulated glycolytic flux by preventing the inhibition of glycolytic enzymes such as glycogen phosphorylase and phosphofructokinase-1 [5,37]. While blood lactate is only an indirect measure of glycolytic flux, our ~2.5 mmol·L −1 difference was comparable to studies reporting greater Yo-Yo IR2 performance for NaHCO 3 [15,16]. Additionally, ergogenic effects of SB-ORAL could be explained by altered electrolyte concentration. SB-ORAL significantly increased blood Na + and reduced blood K + concentrations compared to PLA pre-warm-up, which agrees with previous findings [11,12]. In theory, these changes may lead to upregulation of Na + /K + -ATPase activity that would increase excitationcoupling contraction and sustain force generating capacity of muscles [8,12]. Since exercise-induced losses in intramuscular K + are one of the causes of depressed muscle excitability [8], attenuating reductions in intracellular K + might be crucial for the ergogenic effects for NaHCO 3 . Interestingly, blood K + was not significantly lower post-Yo-Yo IR2 for SB-ORAL compared to PLA. Participants covered greater distance for SB-ORAL, therefore we cannot disregard that a significant effect would have been shown if total work had been matched between conditions. It is also important to note that our results only reveal changes occurring within extracellular compartments, and not whether NaHCO 3 altered electrolyte concentrations within contracting muscles. No significant effects of SB-LOTION were observed for blood acid-base balance or electrolytes. The small, non-significant effect of SB-LOTION on blood lactate after the repeated sprint test (+1.01 mmol·L −1 ) may have contributed to performance benefits, whereas post-Yo-Yo IR2 blood pH was slightly elevated (+0.03 au.) for SB-LOTION compared with PLA despite participants covering more distance. Although not practically feasible, it is possible that a greater dose of PR Lotion would have increased the magnitude of any effects. Overall, our results add to findings by McKay et al. [26] and suggest that topical application of 0.9036 g·kg −1 BM PR Lotion is not able to effectively deliver NaHCO 3 molecules into systematic circulation. Whilst we were unable to trace the absorption of NaHCO 3 molecules from PR Lotion across the skin, it is logical that their negative lipophilicity (partition coefficient, −0.82) somewhat restricts their ability to penetrate lipid bilayers in the stratum corneum [38]. Transdermal drug delivery is also only effective when small doses (mg per day) are required [25]. If PR Lotion does allow NaHCO 3 molecules to penetrate the stratum corneum, given the large doses of NaHCO 3 ingested orally (~20-30 g), it is unclear whether enough NaHCO 3 from PR Lotion could reach the bloodstream to elicit ergogenic changes in HCO 3 − [9]. As such, we suggest an alternative mechanism was responsible for faster repeated sprint times during SB-LOTION. One explanation could be a localized cooling sensation from menthol (~0.5%) in PR Lotion [39]. Menthol is a cyclic terpene alcohol that may improve exercise performance by inducing a "cooling" effect to the skin via stimulation of the membrane-bound ion channel transient receptor potential melastatin 8 [39]. PR Lotion contains proportionately less menthol than other topical menthol formulations [40,41], but a similar absolute amount might have been given as we administered considerably more PR Lotion (~0.4 g menthol; assuming average dose for SB-LOTION was ~74 g) than researchers have used when investigating menthol gels (i.e. 2 mL of ~3.5% menthol Biofreeze® gel [41]). We propose that NaHCO 3 molecules in PR Lotion interacted with menthol to form a protective layer over the skin that intensified menthols' "cooling" sensation. Support for this can be drawn from the 43% participants who reported a "strong cooling" effect for SB-LOTION, whereas participants did not suggest a "cooling" sensation for SB-ORAL or PLA despite our placebo lotion also containing ~0.5% menthol. Furthermore, RPE was significantly lower for SB-LOTION compared with PLA during the repeated sprint test, which agrees with previous findings for topically applied menthol [40]. Reductions in RPE after NaHCO 3 indicate ergogenic benefits might be explained by centrally mediated mechanisms [15,42]. Traditionally, this has been underpinned by the deleterious effects of H + on the force generating capabilities of muscles [7], whereby declining pH causes localized pain [42,43]. Ingesting NaHCO 3 likely leads to peripheral alterations (i.e. fewer H + in the muscle) that modulate activation of group III and IV muscle afferents, in turn reducing negative feedback from muscles and sustaining drive to motor neurons [43,44]. We propose that the "cooling" effect from menthol in PR Lotion induced a similar centrally acting mechanism that attenuated localized muscle pain [45], meaning participants' perception of muscle discomfort was lower than their actual degree of muscle fatigue [41], which allowed them to exert greater effort during the repeated sprint test. Future work should attempt to further elucidate potential physiological mechanisms responsible for ergogenic benefits of PR Lotion.
Another novel finding from this study was the positive effect of oral but not topical NaHCO 3 on hydration status, with the changes in plasma volume for SB-ORAL compared with PLA (~8%) similar to previous results [13,46]. Post-exercise urine osmolality and color were also significantly different for SB-ORAL compared with PLA. Ingestion of water alone is relatively ineffective as a hyperhydration strategy, as most fluid is lost via urine [47]. This may explain why lower urine osmolality and lighter urine color were observed for PLA. Adding substances such as NaHCO 3 with a high osmotic load enhances fluid retention by increasing plasma osmolality and volume [46,47]. Interestingly, differences between supplementation strategies and our participants' high body masses meant that the amount of Na + consumed during SB-ORAL (~6.5 g; proportion of Na + in NaHCO 3 was 27%, assuming 1 mol NaHCO 3 is 84 g·mol −1 ) was greater than previous NaHCO 3 studies [13,46] and more than the ~4 g given for traditional hyperhydration aids [47]. Despite the high amount of Na + consumed, we believe that NaHCO 3 can be safely incorporated into hyperhydration strategies of team sport athletes (i.e. American football, soccer) as they can lose ~4-6 g of Na + per hour during training [48,49]. Our results also reinforce that NaHCO 3 ingestion might be an effective strategy for improving fluid retention and exercise performance during challenging thermal conditions [50]. Additional research is needed to determine the reproducibility of changes in hydration status after NaHCO 3 supplementation.
There are methodological limitations that need to be considered when interpreting our results that should be addressed in the future. First, plasma volume changes from our study were based on indirect measures (e.g. hemoglobin and hematocrit). This approach has been used for estimating changes in plasma volume during maximal exercise [13,31,46], but future research investigating the effect of NaHCO 3 on plasma volume expansion should aim to use direct evaluation techniques [51]. Second, the intensified cooling sensation from menthol in PR Lotion contributed toward 50% participants identifying SB-LOTION, which may explain why participants reported significantly higher expectations of positive outcomes for SB-LOTION. Interestingly, however, 71% participants unable to identify SB-LOTION still improved their repeated sprint performance, and therefore it is difficult to conclude that the ergogenic effects of PR Lotion can be attributed solely to greater expectations. Further work is required to determine the most efficacious strategy for blinding PR Lotion during randomized placebo-controlled trials examining sports performance.

Conclusions
Topical NaHCO 3 (0.9036 g·kg −1 BM PR Lotion) improved 8 × 25 m repeated sprint times by ~2% but had no significant effect on Yo-Yo IR2 or CMJ performance compared to a placebo. Oral NaHCO 3 improved Yo-Yo IR2 and repeated sprint performance, which can likely be explained by elevated HCO 3 − buffering, increased glycolytic flux and sustained excitationcontraction coupling. PR Lotion had no significant effect on blood acid-base balance, suggesting that it did not effectively transport NaHCO 3 across the skin into systematic circulation. Therefore, we propose that ~2% improvements in repeated sprint times for PR Lotion can be attributed to an interaction between NaHCO 3 molecules and menthol that intensified the cooling sensation, subsequently reducing participants' perception of effort and allowing them to exert greater physical effort during the 8 × 25 m repeated sprint test.
In conclusion, PR Lotion appears to improve RSA in team sport athletes, but further research is required to elucidate the physiological mechanisms responsible for performance benefits.